I can't get the difference between these ones in the following context. In PEU, Swan is talking about the differences between uses of the present progressive and the going to structure in sentences about the future. What he says is
We often use the present progressive with verbs of movement, to talk about actions which are just starting.
Another use of the going to structure is to predict the future on the basis of present evidence - to say that a future action or event is on the way, or starting to happen
He also provides the following examples:
Look at the sky. It's gonna rain.
Are you coming to the pub?
Could you explain the difference between the phrase using examples?